Which term is the SI base unit used to measure the amount of a substance, defined so that one unit contains as many particles as there are atoms in 12 g of carbon-12?

• A. mole
• B. Avogadro's number
• C. molar mass
• D. average atomic mass

Answer: (A)

Measuring the amount of substance uses a unit that directly counts particles—the mole. The mole is defined so that one mole contains exactly Avogadro’s number of elementary entities, which is the number of atoms in 12 grams of carbon-12. This establishes a concrete bridge between the microscopic world of particles and the macroscopic world of grams. So the term that fits is the mole. Avogadro’s number is the exact count per mole, molar mass is grams per mole, and average atomic mass is a weighted mass of atoms, not the unit for counting amount of substance.